Gas transmitter with selective gas permeable surfaces

ABSTRACT

The invention relates to a gas transmitter ( 1 ) with selective gas permeable surfaces ( 4 ), comprising a support disc ( 3 ), provided with a number of through openings ( 2 ), made from a semiconductor material, a membrane ( 5 ), covering the openings ( 2 ) in the support disc ( 3 ) which forms the selective gas permeable surfaces ( 4 ) and means for controlling the temperature of the membrane ( 5 ). According to the invention, the temperature control means may be simplified whereby the support disc ( 3 ) itself serves as a means for controlling the temperature of the membrane ( 5 ).

The invention relates to a gas transmitter with selective gas permeablesurfaces having the characteristics of patent claim 1.

A gas transmitter of this kind is employed in measuring or analyticalinstruments. It shall be achieved, for example, that light gases areadmitted into a measuring or analytical instrument in a preferred mannerwhereby heavier gases enter in a less preferred manner. The permeabilityof the membrane for lighter gases is known to be temperature dependent.For the purpose of utilising this—for example, for controlling thepermeability—the membrane needs to be equipped with a heater.

From WO 96/41 677 a gas transmitter of the here affected kind is known.In order to heat the gas permeable surfaces, each of the multitude ofpermeable surfaces is equipped with a heating filament. The heatingfilaments are applied by means of methods from the area of thin-filmtechnology (for example, vacuum coating or evaporation methods,photographic lithography, etching). Moreover, the heating filaments needto be electrically contacted for inclusion within an electric circuit.Also the current feed lines leading to each of the filaments need to beapplied to the membrane by reliance on the aforementioned coatingmethods. In all, equipping gas transmitters with heating means isaccording to the state-of-the-art extremely involved. Finally, theheating filaments have the disadvantage that they cover relatively largeareas of the active gas permeation surfaces.

It is the task of the present invention to render the design of a gastransmitter of the here affected kind significantly more simple withrespect to its equipping with temperature control means for its gaspermeable surfaces.

This task is solved through the present invention in that the supportdisc itself serves as a means for controlling the temperature of themembrane. Through the present invention it is possible to dispense withadditional production steps which according to the state-of-the-art arerequired for equipping the transmitters with temperature control means.Moreover, the gas permeable surfaces are free of heating filamentsimpairing the permeation of the gases.

A commercially available silicon wafer is expediently employed as thesupport disc. Support discs made of other materials having theproperties of a semiconductor (for example, germanium, diamond etc.) mayalso be employed.

The membrane consists expediently of quartz, quartz glass or similarmaterials, for example, Pyrex glass. However, usable are also membraneswith selectively acting properties made of a polymer, for example, FEPas is known from DE-A-43 26 267.

Further advantages and details of the present invention shall beexplained with reference to the examples of embodiments depicted in thedrawing FIGS. 1 to 4.

Depicted is/are in

drawing FIGS. 1 a, 1 b a gas transmitter according to the presentinvention

drawing FIGS. 2 and 3 schematically represented instruments with eachone gas transmitter according to the present invention.

In the drawing figures the gas transmitter is designated as 1, thesupport disc equipped with a multitude of transmission openings 2(drawing FIG. 1 b) with 3, and the membrane forming the permeablesurfaces 4 covering the gas transmission openings 2 in the support discis designated as 5 (drawing FIG. 1 b).

Component of the drawing FIG. 1 a is a partly enlarged view of a sectionof the gas transmitter 1, partly by way of a sectional view, partly byway of a top view. Only this illustration makes apparent thetransmission openings 2, the membrane 5 as well as two of the gaspermeation surfaces 4. The thickness of the support disc 3 is in theorder of magnitude of 0.6 mm; the thickness of the membrane 5 amounts toapproximately 6 μm.

According to the idea on which the present invention is based, thesupport disc 3 itself is employed as a resistance heater. To this end itis equipped in the area of opposing sides with metallic electrodes 6, 7which are expediently applied by evaporation coating. The voltagerequired for producing the heating current is applied to theseelectrodes. Electrodes of this type are not absolutely required; in thecase of simpler solutions also contact clips may be employed.

At room temperature the specific resistance of standard silicon discsamounts to approximately 20 MOhm×cm. At a voltage of 1 kV, a heatingcurrent of approximately 3 μA is produced in a disc having a size of 1cm×1 cm×625 μm, this being equivalent to an electric heating power of 3mW. Through this heating power the wafer is slightly warmed therebyreducing the specific resistance of the semiconductor material, so thatthe heating current, respectively the heating power increases for thesame voltage. At a temperature of T=380° C. the specific resistanceamounts to approximately 3.8 Ohm×cm. The specific resistance thusdecreases by seven orders of magnitude when increasing the temperatureby 360° C. Thus there exists the possibility of being able to preciselycontrol the temperature through the electric current.

In drawing FIG. 2 a leakage gas detector 8 serves as an example for aninstrument in which the gas transmitter 1 in accordance with the presentinvention is employed. A leakage gas detector of this kind is basicallyknown from DE-A-43 26 265. Leaking gas, for example, helium enteringthrough the gas transmitter 1 is detected through the pressure increasein the closed chamber 9 behind the gas transmitter 1. In accordance withthe present invention, the support disc 2 serves as a resistance heaterfor the membrane 5, the permeability of which increases for lightergases at increasing temperatures. After the formation of a signal, theheater may be switched off, for example for preventing the entry ofunnecessarily large quantities of helium into the detector system.Practically the detector 8 presented is a pressure gauge, this beingexpressed through the depicted symbol.

Drawing FIG. 3 depicts a controllable calibrated leak 10 of a design asalready described in German patent application 101 22 733.7. Itsubstantially consists of a test gas reservoir 12, a base 14 with a testgas outlet 16 and a control facility 18.

The test gas reservoir 12 is formed by a gas-tight pot-shaped reservoirvessel 20 being inserted with the opening pointing down in a gas-tightmanner into the upper section of the base 14.

The metal base body of the base 14 exhibits an axially verticallyextending outlet channel 17 forming the test gas outlet 16. Embedded atthe end of the outlet channel 17 on the side of the reservoir vessel isan annular step-like shoulder 26 in the base body 15, said shouldersupporting the gas transmitter 1 on an annular insulation body 28.

In the axial centre area of outlet channel 17, a filter disc 43 with asecuring ring 44 is arranged for the purpose of providing a means ofmechanical protection, said filter disc preventing the entry ofparticles into the sensitive analytical instrument downstream.

On the outlet side of the base 14, a mounting flange 46 is providedserving the purpose of being able to easily mount the test leak facility10 to an adjacent element.

The insulation body 28 consists of a heat and gas resistant materialwith good heat insulating properties—and insulated the gas transmitter 1thermally with respect to the base body 15. Thus the dissipation of heatinto the base 14 is reduced to a minimum so that the amount of heatingenergy required for maintaining a certain temperature is as low aspossible. For the purpose of implementing high modulation frequenciesthe insulation body 28, however, may consist of a material with goodheat conducting properties.

With the calibrated leak facility detailed, leakage rates of 10⁻¹¹ to10⁻⁴ mbar x|x s⁻¹ can be implemented.

The described calibrated leak facility 10 represents on the one hand asource of test gas capable of being precisely adjusted and controlledacross a wide range of leakage rates and is simultaneously highlyreliable, since the possibility of blocking the outlet channel 17 or thegas transmitter 1 is practically excluded.

1-9. (canceled)
 10. A gas transmitter with selective gas permeablesurfaces comprising: a support disc having a plurality of throughopenings, said disc being made from a semiconductor material; a membranecovering the openings in the support disc, thereby forming selective gaspermeable surfaces; and means for controlling the temperature of themembrane wherein the support disc itself serves as a means forcontrolling the temperature of the membrane.
 11. A gas transmitteraccording to claim 10, wherein in the support disc consists at leastmostly of silicon.
 12. A gas transmitter according to claim 10, whereinthe membrane consists of a material consisting of at least one of thegroup of quartz, quartz glass and Pyrex glass.
 13. A gas transmitteraccording to claim 10, wherein the membrane consists of a polymer.
 14. Agas transmitter according to claim 10, wherein the support disc isequipped in areas substantially facing each other with electrodeswhereby said support disc itself is serves as a resistance heater.
 15. Agas transmitter according to claim 10, wherein said transmitter is partof a helium leakage gas detector.
 16. A gas transmitter according toclaim 10, wherein said transmitter is a component of a controllablehelium calibrated leak.
 17. A leakage gas detector in which at least oneleakage gas is detected through a pressure rise, wherein said leakagegas detector is equipped with a gas transmitter with selective gaspermeable surfaces, said gas transmitter comprising: a support dischaving a plurality of through openings, said disc being made from asemiconductor material; a membrane covering the openings in the supportdisc, thereby forming selective gas permeable surfaces; and means forcontrolling the temperature of the membrane wherein the support discitself serves as a means for controlling the temperature of themembrane.
 18. A controllable calibrated leak in which the leakage rateis adjustable with the aid of a selectively permeable membrane, whereinsaid controllable calibrated leak is equipped with a gas transmitterwith selective gas permeable surfaces, said gas transmitter comprising:a support disc having a plurality of through openings said disc beingmade from a semiconductor material; a membrane covering the openings inthe support disc thereby forming selective gas permeable surfaces; andmeans for controlling the temperature of the membrane wherein thesupport disc itself serves as a means for controlling the temperature ofthe membrane.
 19. A controllable leak according to claim 18, wherein theleak is preferably for helium.